Welding Techniques Produce Faster Travel Speeds and Improve Mechanical Properties
As a key supplier to the Gulfstream Pipe Project, Berg Steel Pipe Corporation of Panama City, Florida is challenged with producing high-quality, defect-free linepipe at top levels of productivity. To do this, the company employs a number of innovative welding systems, which includes a continuous seam tacking system as well as inside diameter (ID) and outside diameter (OD) pipe welding stations that use multiple wire welding techniques.
In comparison to two-wire submerged arc welding (SAW) systems, Berg's techniques have created welding travel speeds that are three times greater for welding the ID and four times greater for OD welding on 0.820" thick pipes. In addition, total heat input has been decreased by 25 percent while overall mechanical weld properties have been improved by these systems. Overall, employing multiple-wire welding has enabled Berg Steel to produce 40 foot long 0.820" thick pipes at a rate of 14 pipes per hour.
During 2001, Berg manufactured for the Gulfstream Project, a natural gas pipeline that originates near Pascagoula, Mississippi and crosses the Gulf of Mexico to Manatee County, Florida. Once onshore, the pipeline stretches across south and central Florida to Palm Beach County. This natural gas pipeline will serve Florida utilities and power generation facilities, generating 1.1 billion cubic feet per day of additional natural gas - enough to supply electricity for 4.5 million homes.
For Phase One of this project, the offshore portion, Berg Steel Pipe along with its parent company, Europipe GmbH in Germany, supplied 443 miles of pipe in only six months. In Phase Two, Berg will produce 292 more miles for the onshore pipeline.
|A 40 foot long sheet is being loaded into the planer for edge preparation.|
Berg Pipe's welding systems have been supplied by The Lincoln Electric Company and its German subsidiary and system designer, Uhrhan & Schwill. These systems help Berg achieve high production levels. It also makes the company hard to match in terms of welding technology and innovation.
"Welding technology has allowed us to transform a facility that was built in 1980 to produce 5,000 tons of pipe per month to a facility that produced more than 40,000 tons per month during the Gulfstream Project," said John Burton, General Manager of Production, Berg Steel Pipe Corporation. "We needed to add the multiple-wire processes because we don't have enough square footage to add more welding stations. We have to make use of the technology on the market to be the most efficient plant possible in the space we have available."
Berg is able to service customers that range from oil and gas transmission companies to pipe distributors. The 275-employee company also manufactures structural piling and is even qualified to produce linepipe to arctic specifications with weld metal Charpy requirements to -50° F. Berg Steel Pipe Corporation can supply to this broad range of customers because of the flexibility of its three-roll bending type forming process, which offers a unique advantage to do quick changeovers so the company can accept both small and large projects.
Not only does project size vary, but so does the type of pipe produced. Berg can manufacture in diameters ranging from 24" through 64" and wall thicknesses from 0.250" to 1.500". Grades that can be handled at the Berg facility are API Grade B through X80.
For purposes of this article, we will look at the pipe produced for the Gulfstream Project in particular, and the productivity and efficiencies achieved during the welding of the pipe as it proceeded through Berg's mill. The majority of the offshore linepipe was 36" diameter, 0.820" (20.8 mm) thick API 5L Grade X70 produced in 40-foot nominal lengths.
Preparation of Plates
Before welding, Berg planes the plates to the precise required width and bevels the plate edges for longitudinal seam welding. This beveling operation prepares plate edges with a double V groove joint design for the upcoming welding operations. After plate edge preparation, the plate is formed into a rough tubular shape by a three-roll bending machine (pyramid roll). Final tubular forming is subsequently completed by rounding the longitudinal edges of the plate between a hydraulically cushioned top ball and matching hourglass roll.
Once the plate is formed into the proper tubular shape, the longitudinal weld is completed in three passes, beginning with a continuous tack weld. For the Gulfstream Project, the longitudinal welding procedures were qualified to both ASME Section IX and Berg's internal standards.
The 40 foot long sheet is formed into a tube by Berg's Tri-Roll Bender.
The tack welding system is designed to secure the welding edges in proper alignment for welding and to provide continuous backing for subsequent inside submerged arc welding. This process is completed at Berg Steel Pipe using a welding system purchased two years ago to remedy a bottleneck the company was experiencing in this stage of the manufacturing process.
In 1999, Berg had a single, DC 1500 power source. For every diameter change in pipe, the system would require 45 minutes to change out the cage that positioned the plate edges for welding. To eliminate this, Berg turned to the expertise of The Lincoln Electric Company. Because of a long, 21-year relationship with Lincoln, Berg looks to the company for innovative solutions to welding challenges and ways to provide higher productivity in its welding operations. At Lincoln's suggestion, Berg installed a continuous tack welding system utilizing two, DC 1000 power sources and a NA-5 control. This new, CNC-controlled tack welder has hydraulic cylinders, which automatically reposition themselves when the pipe diameter changes. This system has reduced changeover time to approximately five minutes. This new system has also eliminated some of the problems with closing the bevel for welding that Berg Pipe was experiencing with its former system.
|The formed tube is tack-welded at the laser-guided continuous tack welding station.|
These two DC 1000s create a continuous seam tack using an open arc gas-metal arc welding (GMAW) process at high speeds of 260 inches per minute. Commonly referred to as "buried arc", this process is characterized by low voltage, short arc length, and very high travel speeds allowing for a deep penetrating weld at reduced heat inputs.
During tack welding, hydraulic rollers on the system hold the pipe seam together. A laser guidance system from Uhrhan & Schwill is also used to guide the arc in the weld groove and maintain radial alignment of the plate edges. Because this laser guidance was not present on the old tack welding system, it eliminates the time that was needed previously for the operator to stop the weld and adjust these items manually.
While some competitors use intermittent tack welds, Berg Steel Pipe Corporation feels there are benefits to a continuous seam technique to close the formed pipe cylinder for welding. "Intermittent tack welds require the use of a backing flux or a copper backing bar during subsequent submerged arc welding, while ours does not," noted Fred Hafner, PE, Chief Metallurgist/QA Manager for Berg Steel Pipe Corporation. "This means that we achieve higher productivity since the backing provided by the continuous seam provides the attributes for fast travel speeds and deep penetration in ID welding."
Using a buried arc process allows for the elimination of weld spatter typical of globular transfer because the arc is "buried" in the weld puddle. This process also requires only a CO2 shielding gas, eliminating the need for more expensive argon gas.
For the tack welding of 0.820" thick pipe, Berg Steel Pipe Corporation uses Lincoln L-50™ 5/32" diameter wire electrode at 1,500 amps.
Since installing the new system, Berg has been extremely pleased with the results. "Our tacking system is very reliable and helped relieve the problem of the arc outage we were experiencing," said Burton. "We also employ the use of CNC controls which can store data on a particular wall thickness so that we can provide rapid changeovers by simply calling up previously-stored information."
Inner Diameter Welding
After the tacking process is complete, weld tabs are added to the seam areas at both ends of the pipe with a manual GMAW process. This allows the ID and OD systems to start and end welding on the run-off tabs, rather than on the pipe itself. In this way, the large pile of weld metal created at the start of this multiple-wire process will not affect the pipe weld nor will the concave area left at the end of the weld. This tab will be removed after welding so that what is left on the pipe is the best possible weld and not the undesirable weld metal from the start or end of welding.
Berg has three, four-wire ID welding systems utilizing Lincoln AC 1200 power sources with NA-4 controllers. This SAW process, run at up to 1,000 amps, is able to produce a large-sized bead all in one pass. Although there is one power source per arc, all four wires are arranged in-line and feed into a single puddle. Since in multiple-arc welding one arc may be deflected by the others, it is necessary to ensure that such deflections are favorable to the weld profile. At Berg, ID submerged arc welding is conducted with constant-current AC power sources arranged such that incoming AC power is successively 90 degrees out of phase.
During inside welding, the pipe is moving forward while all the welding equipment is fixed and mounted to a boom that permits travel through the length of the 40-foot pipe. The arcs are buried underneath the granulated flux.
For the ID weld, a V-shaped guide wheel rides in the weld seam groove to ensure alignment of the weld head in the weld groove. The ID weld penetrates into the previously laid continuous tack weld that is now serving as weld backing at this stage.
The four-wire process on the ID weld creates faster travel speeds - up to 68 ipm on the Gulfstream pipe- and the mechanical properties of the weld improve because there is less heat input delivered per arc.
Depending on customer specifications, different types of wires are used at this station. For the Gulfstream Project, 1/8" Lincoln L-61 with a 761 pipe flux are the consumables of choice. This special 761 flux is a special adaptation of the standard Lincoln 761 flux with different-sized particles to create better fluidity, bead shape and edge tie-in to fill in the weld groove on the inside of the pipe.
In the last step of the welding process, Berg uses two, five-wire SAW stations to create OD welds that penetrate into the ID and completely consume the tack weld.
Finished pipe is loaded on the barge for transport to the lay barge.
There are three main advantages to using a five-wire process: 1) higher weld speeds - the process is four times the speed of the standard, two-wire process; 2) reduced heat input, which reduces grain coarsening in the heat affected zone; and 3) impurities such as slag or porosity have time to escape prior to solidification because of the large weld puddle.
The Berg OD systems use a DC lead wire to ensure complete penetration while the trail arcs are AC for fill and travel speed. At one welding cell, Berg uses two Lincoln DC-1000 units in parallel with Lincoln NA-3 controls. At another welding cell, two Lincoln DC-1500 units are used with similar NA-3 controls. In all cases, the AC trailing wire is produced with AC-1200 units from Lincoln and NA-4 controls.
Laser seam tracking on the OD ensures that the weld is positioned correctly in the weld groove. Since the arc is buried under a pile of flux and alignment can't be seen, this equipment facilitates positioning of the weld head.
For the 0.820" (20.83 mm) thick Gulfstream pipe, the 3/16" DC lead wire operates at 1,550 amps with a weld speed of 90 ipm. Welding consumables were a combination of L-61 and L-70 wire and 995 flux. (L-70 wires are used when higher weld metal Charpy impacts are required - the addition of 0.5 percent Mo L-70 wire results in higher acicular ferrite contents in the weld, and this improves the weld metal Charpy impact energy.)
On pipe thinner than 14.3 mm (0.56”), a four-wire process is used while all five-wires are utilized for heavier wall thicknesses.
As customers requirements dictate changing wire or flux, Berg Steel Pipe Corporation has called on Lincoln Electric to help run trials at the company's Cleveland headquarters where a five-wire welding station is located for this purpose. In this way, valuable time on Berg's machines doesn't have to be spent on product development.
After welding, the weld tabs are removed manually by operators using oxy-acetylene cutting. Slag is removed the length of the longitudinal weld, typically with a wire brush.
Berg Pipe Corporation then sizes the pipe ends between the opposing hourglass rolls to ensure uniform diameters and roundness essential for efficient field constructions. Next, the company simultaneously bevels both ends of the pipe for field girth welding. A stenciling machine labels and barcodes each pipe with its unique identification before shipping via barge or rail to Gulfstream.
Because creating defect-free pipe is so important for high-pressure, natural gas applications, pipes are subject to many nondestructive tests. Testing at Berg Steel Pipe includes automated ultrasonic inspection of the weld seam and film radiography of the seam weld near the pipe ends. Per API 5L, Grade X70 pipes are hydrostatically tested for a minimum duration of ten seconds at a pressure equivalent to at least 90 percent of the specified minimum yield strength. During the Gulfstream project, pipes were tested to higher pressures – for a minimum of 20 seconds at a pressure equivalent to the full 100 percent of the specified minimum yield strength. In addition, dimensional inspection and internal and external visual inspections are completed on each length of pipe.
According to Burton, welding the pipe correctly the first time is critical to achieving high production rates. "In our pipe mill, a pipe that needs rework has to displace a new pipe in the production process," said Burton. "Removing or holding up a new pipe from production to correct a problem can be very costly."
"We are absolutely pleased with Lincoln Electric and the high level of service we get from our local representative, Troy Gurkin," noted Hafner. "He is always available if we have a problem and we can also refer to pipe welding specialists at Lincoln headquarters. Another advantage we get from Lincoln is that the company keeps a safety stock of consumables in Atlanta. This means that we always have an ample supply nearby."
Each finished pipe is hydrostatically pressure-tested to assure the integrity of the welding.
Berg Steel Pipe plans to continue its long relationship with Lincoln. "As the company grows and moves into new areas, Lincoln will be directly involved in all phases," explained Burton. Currently, the company is looking to develop seam welding procedures and process controls for Grade X80 pipe. Berg recognizes that Lincoln's Surface Tension Transfer® (or STT®) process will have advantages for field welding of this grade and will help popularize this grade for new pipelines.
"Berg is one of the most progressive companies that I have worked with," noted Troy Gurkin, Lincoln Electric area sales representative. "The company is always looking to evaluate new products which can help increase efficiency."
Several of Berg's employees have even attended preventative maintenance seminars presented at the local Lincoln office in Birmingham.
"In the future we expect SAW systems to be replaced with software-controlled inverter welding systems that will allow complete manipulation of the waveform and electronic phasing. We will then be able to change the waveform for each wire," said Hafner. "Such systems will provide even better control of weld penetration and arc deflection with higher productivity."
Multiple wire submerged arc welding of the inside of the pipe.
Originally Written 1/31/2002